The largest cities in the Rhine-Ruhr area are Cologne, with over 1 million inhabitants, followed by Düsseldorf, Dortmund and Essen, which each have a population of over 500,000.

There are many different sub-definitions of what belongs to the Rhine-Ruhr area, but the metropolitan area itself has officially defined borders with Hamm in the east, Mönchengladbach in the west and Bonn in the south and the small city Wesel as its northernmost point. The northern border is similar to the border of the Ruhr Area.

The table below shows an unofficial summary of regions. In the official definition the metropolitan area is much smaller.

Historically, most of the Ruhr area was for the most part characterized by heavy industry since the age of industrialisation in the late 19th and early 20th century. Since the Middle Ages, Cologne, Dortmund and other cities were important regional trading cities, but during the 19th century the city of Düsseldorf grew to become the administrative center of the region and since 1945 its political capital.

The rail, S-Bahn, U-Bahn and bus companies are administered through a consortium of local and regional transport liness, the Verkehrsverbund Rhein-Ruhr. It offers a rapid transit system which interconnects all cities and their respective local buses, trams, U- and S-Bahn systems, partly under the umbrella of Deutsche Bahn. Their systems are highly integrated where even some subway lines continue from one city to the next (for example between Düsseldorf and Duisburg or Bochum and Herne, which is unique in germany, as the city border is crossed underground). The region is divided into several urban zones and fares are paid according to the amount of urban areas (or zones) passed through. Tickets include door to door transportation with all forms covered in one ticket with the exception of high speed rail (which only stops in the major cities). Some excursions, theatre and opera tickets as well as museums offer free transportation from any point in the Rhine Ruhr area to the venue and return.

The following register lists all municipalities that officially belong to Rhine-Ruhr area. Demographically, these municipalities include 20 cities (German: ″Kreisfreie Städte″), each with more than 100,000 inhabitants, and 11 districts (German: "Kreis"), each with a population of more than 250,000 inhabitants. Some districts only belong partly to Rhine-Ruhr area. In such a case only the municipalities that belong to the metro area are listed.